Piezoelectric transformer drive method and drive circuit

ABSTRACT

A driving circuit is composed of a discontinuous rectangle wave oscillator and a lowpass filter. A discontinuous rectangle wave used herein is composed of voltages Vp, 0 and −Vp. Assuming one cycle is T, a discontinuous rectangle wave has such a configuration that a potential from a time 0 to a time ø is a potential 0, from the time ø to a time (T/6−ø) is a potential Vp, from the time (T/6−ø) to a time (T/3+ø) is a potential 0, . . . . Since the third harmonic wave is not included in the discontinuous waveform, the inductance of the lowpass filter can be reduced. Consequently, the lowpass filter may be omitted. Further, the amplitude of the fundamental wave can be changed by varying the phase difference ø.

TECHNICAL FIELD

The present invention relates to a driving method and a driving circuitof a piezoelectric transformer which transforms AC voltage by utilizinga resonance phenomenon of a piezoelectric vibrator.

BACKGROUND ART

A piezoelectric transformer (solid former) is configured to input lowvoltage and output high voltage by utilizing a resonance phenomenon of apiezoelectric vibrator (see, for example, Patent Document 1). Acharacteristic of a piezoelectric transformer is that the energy densityof a piezoelectric vibrator is higher comparing with an electromagnetictype. Therefore, the size can be reduced, whereby piezoelectrictransformers are used for cold-cathode tube lighting, liquid crystalbacklight lighting, a small AC adapter, a small high-voltage powersource, and the like.

FIG. 8 shows a piezoelectric transformer, in which FIG. 8[1] is aperspective view, FIG. 8[2] is a side view, and FIG. 8[3] is anequivalent circuit diagram. Hereinafter, explanation will be given basedon these Figs.

A piezoelectric transformer 10 includes primary electrodes 12 and 13 anda secondary electrode 14 on a piezoelectric vibrator 11, and the primaryside is polarized in a thickness direction (arrow 15) and the secondaryside is polarized in a length direction (arrow 16). The primaryelectrodes 12 and 13 are opposite each other sandwiching thepiezoelectric vibrator 11. The piezoelectric vibrator 11 is in a plateshape (rectangular parallelepiped shape) having the length L, the widthW and the thickness t. In the length direction of the piezoelectricvibrator 11, the primary electrodes 12 and 13 are provided from one endto L/2 in a width direction, and the secondary electrode 14 is providedin a thickness direction on the other end. When the voltage of a naturalresonance frequency fr determined by the length dimension is inputtedinto the primary side of the piezoelectric transformer, intensemechanical vibration is caused in the piezoelectric transformer due tothe inverse piezoelectric effect of the piezoelectric transformer, andwith the piezoelectric effect thereof, an intense voltage Vo appropriateto the vibration is outputted to the secondary side.

The distribution of displacement and stress at the time of driving thepiezoelectric transformer 10 is shown in FIG. 8[2]. Parts holding thepiezoelectric vibrator 11 are nodes located at positions which are ¼ ofthe length from both ends in the case of λ mode. An equivalent circuitin the vicinity of the resonance frequency fr of the piezoelectrictransformer 10 can be written as shown in FIG. 8[3].

[Patent Document 1]

Japanese Patent Application Laid-open No. 8-32134

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

FIG. 9 shows a driving circuit of a conventional piezoelectrictransformer, in which FIG. 9[1] is a functional block diagram, and FIG.9[2] shows an output waveform of a rectangle wave oscillator.Hereinafter, explanation will be given based on FIGS. 8 and 9.

A driving circuit 90 basically consists of a rectangle wave oscillator91 and a lowpass filter 92. The lowpass filter 92 removes harmoniccomponents included in the rectangle wave voltage outputted from therectangle wave oscillator 91 to thereby apply a waveform close to a sinewave to the piezoelectric transformer 10.

Fourier series of a rectangle wave (or may be called a square wave)shown in FIG. 9[2] is given by the following formula [2].(4Vp/π) [sin ωt+(⅓)sin 3ωt+(⅕)sin 5ωt+ . . . +{1/(2n+1)}sin(2n+1)ωt+ . .. ]  [2]

As obvious from the formula [2], harmonic waves other than thefundamental wave included in the rectangle wave are removed, so thecutoff frequency of the lowpass filter 92 is generally set to the thirdharmonic wave. Since such a low cutoff frequency is required to be set,the inductance of the lowpass filter 92 must be large. However, there isa problem that an inductor having a large inductance is large in itsdimensions, heavy and expensive.

Further, from the formula [2], the fundamental wave is shown as(4Vp/π)sin ωt. Therefore, in order to change the amplitude of thefundamental wave, a DC-DC converter for causing the voltage Vp to bevariable is needed, which makes the configuration complicated.

In view of the above, an object of the present invention is to provide adriving method and a driving circuit of a piezoelectric transformer,capable of reducing the inductance of a lowpass filter and also changingthe amplitude of the fundamental wave without using a DC-DC converter.

Means for Solving the Problems

A driving method of a piezoelectric transformer according to the presentinvention is to apply the voltage of a discontinuous rectangle wave toprimary electrodes of the piezoelectric transformer. A driving circuitof a piezoelectric transformer according to the present invention is onehaving a discontinuous rectangle wave oscillator which outputs thevoltage of a discontinuous rectangle wave which is applied to primaryelectrodes of the piezoelectric transformer.

A discontinuous rectangle wave used herein has the followingcharacteristics [1] to [3]. [1] It consists of a potential V₀, apotential V_(H) which is higher than the potential V₀, and a potentialV_(L) which is lower than the potential V₀. [2] Assuming one cycle is T,the discontinuous rectangle wave has a such configuration that apotential from a time 0 to a time ø is the potential V₀, from the time øto a time (T/6−ø) is the potential V_(H), from the time (T/6−ø) to atime (T/3+ø) is the potential V₀, from the time (T/3+ø) to a time(T/2−ø) is the potential V_(L), from the time (T/2−ø) to a time (T/2+ø)is the potential V₀, from the time (T/2+ø) to a time (2T/3−ø) is thepotential V_(L), from the time (2T/3−ø) to a time (5T/6+ø) is thepotential V₀, from the time (5T/6+ø) to a time (T−ø) is the potentialV_(H), and from the time (T−ø) to a time T is the potential V₀. [3] Thephase difference ø is 0≦ø≦T/12.

It is preferable that a discontinuous rectangle wave used herein has thefollowing characteristics [1] to [3]. [1] It consists of a potential 0,a potential +1 which is higher by a certain voltage than the potential0, and a potential −1 which is lower by a certain voltage than thevoltage 0. [2] Assuming one cycle is T, the discontinuous rectangle wavehas a such configuration that a potential from a time 0 to a time ø isthe potential 0, from the time ø to a time (T/6−ø) is the potential +1,from the time (T/6−ø) to a time (T/3+ø) is the potential 0, from thetime (T/3+ø) to a time (T/2−ø) is the potential −1, from the time(T/2−ø) to a time (T/2+ø) is the potential 0, from the time (T/2+ø) to atime (2T/3−ø) is the potential −1, from the time (2T/3−ø) to a time(5T/6+ø) is the potential 0, from the time (5T/6+ø) to a time (T−ø) isthe potential +1, and from the time (T−ø) to a time T is the potential0. [3] The phase difference ø is 0≦ø≦T/12.

As obvious from the formula [2], assuming the coefficient of thefundamental wave is 1, a rectangle wave (conventional art) consists ofthe fundamental wave+(⅓)third harmonic wave+(⅕)fifth harmonic wave+ . .. . That is, in a rectangle wave, the third harmonic wave occupies themost among the harmonic waves. Thereby, a lowpass filter in which thethird harmonic wave is the cutoff frequency is required.

On the other hand, a discontinuous rectangle wave (present invention)does not include the third harmonic wave as described later, so theinductance of the lowpass filter can be small. For example, theinductance of the lowpass filter in which the fifth harmonic wave is thecutoff frequency is sufficient. Further, as described later, theamplitude of the fundamental wave can be changed by varying the phasedifference ø, whereby a DC-DC converter is not required. Note thatassuming the coefficient of the fundamental wave is 1, a discontinuousrectangle wave when ø is 0 is so configured as the fundamentalwave+(−⅕)fifth harmonic wave+( 1/7)seventh harmonic wave+ . . . .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a driving circuit of a piezoelectrictransformer according to the present invention, in which FIG. 1[1] is afunctional block diagram, and FIG. 1[2] shows an output waveform of adiscontinuous rectangle wave oscillator.

FIG. 2 is a graph showing the relationship between the phase differenceand the coefficient of the fundamental wave in the driving circuit ofFIG. 1.

FIG. 3 is a graph showing harmonic components of a rectangle wave(conventional art) and a discontinuous rectangle wave (presentinvention).

FIG. 4 is a circuit diagram showing an example of an oscillating unit inthe driving circuit of FIG. 1.

FIG. 5 is a timing chart (No. 1) showing respective output signals inthe oscillating unit of FIG. 4.

FIG. 6 is a timing chart (No. 2) showing respective output signals inthe oscillating unit of FIG. 4.

FIG. 7 is a circuit diagram showing an example of a driving unit in thedriving circuit of FIG. 1.

FIG. 8 shows a piezoelectric transformer, in which FIG. 8[1] is aperspective view, FIG. 8[2] is a side view, and FIG. 8[3] is anequivalent circuit diagram.

FIG. 9 shows a driving circuit of a conventional piezoelectrictransformer, in which FIG. 9[1] is a functional block diagram, and FIG.9[2] shows an output waveform of a rectangle wave oscillator.

Hereinafter, as for a driving method and a driving circuit of apiezoelectric transformer according to the present invention,embodiments thereof will be explained with reference to the drawings.Since the driving method according to the present invention is used forthe driving circuit according to the present invention, it will beexplained together with the explanation about an embodiment of thedriving circuit according to the present invention.

FIG. 1 shows an embodiment of a driving circuit of a piezoelectrictransformer according to the present invention, in which FIG. 1[1] is afunctional bock diagram, and FIG. 1[2] shows an output waveform of adiscontinuous rectangle wave oscillator. FIG. 2 is a graph showing therelationship between the phase difference and the coefficient of thefundamental wave in the driving circuit of FIG. 1. FIG. 3 is a graphshowing harmonic components of exemplary rectangle wave (conventionalart) and discontinuous rectangle wave (present invention). Hereinafter,explanation will be given based on FIGS. 1 to 3. Since the piezoelectrictransformer is the same as that of the conventional art, the explanationis omitted.

As shown in FIG. 1[1], a driving circuit 20 of the present embodimentconsists of a discontinuous rectangle wave oscillator 21 and a lowpassfilter 22. The lowpass filter 22 is provided between the rectangle waveoscillator 21 and the piezoelectric transformer 10. The lowpass filter22 removes harmonic components included in the discontinuous rectanglewave voltage outputted from the discontinuous rectangle wave oscillator21 to thereby apply a waveform close to a sine wave to the piezoelectrictransformer 10. Note that the discontinuous rectangle wave oscillator 21includes an oscillating unit (FIG. 4) for outputting discontinuousrectangle wave generation signals, and a driving unit (FIG. 7) forapplying voltage generated by a discontinuous rectangle wave based onthe discontinuous rectangle wave generation signals to primaryelectrodes of the piezoelectric transformer 10.

The driving method of a piezoelectric transformer according to thepresent invention is characterized in that, as the basic configuration,in a pulse generating step, the driving pulse of the fundamental wave isgenerated by alternating the polarities of rectangle pulses 100 a, 100b, 101 d and 101 e, in a double-humped shape, forming a unit, and thenin a voltage applying step, a voltage based on the driving pulse of thefundamental wave generated in the pulse generating step is applied toprimary electrodes of the piezoelectric transformer 10. Note that thedriving pulse in which the amplitude is changed may be generated byvarying the phase difference (ø) with respect to the fundamental wave.Further, frequency components not less than the fifth harmonic wave,included in the driving pulse, may be filtered.

A driving circuit of a piezoelectric transformer for carrying out thedriving method of a piezoelectric transformer according to the presentinvention includes an oscillating unit (FIG. 4) and a driving unit (FIG.7).

As shown in FIG. 1[2], the oscillating unit (FIG. 4) generates thedriving pulse of the fundamental wave by alternating the polarities ofthe rectangle pulses 100 a and 100 b, and 100 d and 100 e, in adouble-humped shape, forming a unit. Note that the oscillating unit(FIG. 4) has a function of generating the driving pulse in which theamplitude is changed by varying the phase difference (ø) with respect tothe fundamental wave.

Based on FIG. 1[2], the rectangle pulses 100 a and 100 b, and 100 d and100 e, in a double-humped shape, constituting the driving pulse of thefundamental wave will be explained. The rectangle pulses 100 a, 100 b,100 d and 100 e are double-humped shaped, and the rectangle pulses 100 aand 100 b and the rectangle pulses 100 d and 100 e are different intheir polarities. That is, the polarity of the rectangle pulses 100 aand 100 b show positive, and the rectangle pulses 100 d and 100 e shownegative. The rectangle pulses 10 a and 10 b, in a double-humped shape,have a voltage value of +Vp, and the voltage value of a rectangle pulse100 c between the rectangle pulses 100 a and 100 b, in a double-humpedshape, is 0. On the other hand, the rectangle pulses 100 d and 100 e, ina double-humped shape, have the voltage value of −Vp, and the voltagevalue of a rectangle pulse 100 f between the rectangle pulses 100 d and100 e, in a double-humped shape, is 0.

The driving unit (FIG. 7) applies a voltage based on the driving pulseof the fundamental wave generated in the oscillating unit (FIG. 4) toprimary electrodes of the piezoelectric transformer 10. Note that thelowpass filter 22 for filtering frequency components not less than thefifth harmonic wave included in the driving pulse may be provided.

Further, the driving pulse as a discontinuous rectangle wave will beexplained in detail. As shown in FIG. 1[2], a discontinuous rectanglewave (driving pulse) used here has the characteristics shown in [1] to[3] below. [1]. It consists of potentials Vp, 0, −Vp. [2]. Assuming onecycle is T, it has such a configuration that the potential from a time 0to a time ø is a potential 0, from the time ø to a time (T/6−ø) is apotential Vp, from the time (T/6−ø) to a time (T/3+ø) is a potential 0,from the time (T/3+ø) to a time (T/2−ø) is a potential −Vp, from thetime (T/2−ø) to a time (T/2+ø) is a potential 0, from the time (T/2+ø)to a time (2T/3−ø) is a potential −Vp, from the time (2T/3−ø) to a time(5T/6+ø) is a potential 0, from the time (5T/6+ø) to a time (T−ø) is apotential Vp, and from the time (T−ø) to a time T is a potential 0. [3].ø is 0<ø<T/12. As described above, the driving circuit 20 uses thedriving method according to the present invention.

Next, the discontinuous rectangle wave shown in FIG. 1[2] is expressedin a function y(t) of the time (t), and is developed into the Fourierseries. If ω=2π/T, y(t) is shown as follows:y(t)=b ₀+Σ_(n=1) ^(∞) b _(n) cos nωt+Σ _(n=1) ^(∞) a _(n)sinnωt  (1)

Here, as shown in FIG. 1[2], since y(t+T/2)=−y(t) is established, y(t)is a symmetrical wave. Therefore, b₀ is 0, and n is an odd number only,and integration may be from 0 to T/2. Further, since y(t)=−y(t) isestablished, y(t) is an even function, whereby the term of sin does notexist in the equation (1). That is, a_(n) is 0.

Therefore, y(t) can be shown as follows:y(t)=Σ_(n=1) ^(∞) b _(n)cosnωt  (2)

Next, the coefficient b_(n) in the equation (2) is calculated.

$\begin{matrix}\begin{matrix}{b_{n} = {2/{\prod{\int_{0}^{T/2}{{y(t)}\cos\; n\;\omega\; t\ {\mathbb{d}t}}}}}} \\{= {2/{\prod\left( {{\int_{\phi}^{{\prod{/3}} - \phi}{{Vp}\;\cos\; n\;\omega\; t\ {\mathbb{d}t}}} - {\int_{{2{\prod{/3}}} + \phi}^{\prod{- \phi}}{{Vp}\;\cos\; n\;\omega\; t\ {\mathbb{d}t}}}} \right)}}} \\{= {2{{Vp}/\left( {n\prod} \right)}\left( {\left\lbrack {\sin\;\omega\; t} \right\rbrack_{\phi}^{{\prod{/3}} - \phi} - \left\lbrack {\sin\;\omega\; t} \right\rbrack_{{2{\prod{/3}}} + \phi}^{\prod{- \phi}}} \right)}} \\{= {2{{Vp}/\left( {n\prod} \right)}\begin{Bmatrix}{{\sin\left( {{n{\prod{/3}}} - {n\;\phi}} \right)} - {\sin\; n\;\phi} - \sin} \\{\left( {n{\prod{{- n}\;\phi}}} \right) + {\sin\left( {{2n{\prod{/3}}} + {n\;\phi}} \right)}}\end{Bmatrix}}}\end{matrix} & (3)\end{matrix}$

From the equation (3), b₁, b₃, b₅, . . . are derived as follows.

$\begin{matrix}\begin{matrix}{b_{1} = {2{{Vp}/{\prod\begin{Bmatrix}{{\sin\left( {{\prod{/3}} - \phi} \right)} - {\sin\;\phi} -} \\{{\sin\;\left( {\prod{- \phi}} \right)} + {\sin\left( {{2{\prod{/3}}} + \phi} \right)}}\end{Bmatrix}}}}} \\{= {4{{Vp}/{\prod\left\{ {{\sin\left( {{\prod{/3}} - \phi} \right)} - {\sin\;\phi}} \right\}}}}}\end{matrix} & (4)\end{matrix}$

∵sin(π−ø)=sinø,

-   -   sin(2π/3+ø)=sin{π−(π/3−ø)}=sin(π/3−ø)

$\begin{matrix}\begin{matrix}{b_{3} = {2{{Vp}/\left( {3\prod} \right)}\begin{Bmatrix}{{\sin\left( {\prod{{- 3}\phi}} \right)} - {\sin\; 3\phi} -} \\{{\sin\left( {3{\prod{{- 3}\phi}}} \right)} + {\sin\left( {2{\prod{{- 3}\phi}}} \right)}}\end{Bmatrix}}} \\{= {2{{Vp}/\left( {3\prod} \right)}\left\{ {{\sin\; 3\;\phi} - {\sin\; 3\;\phi} - {\sin\; 3\;\phi} + {\sin\; 3\phi}} \right\}}} \\{= 0}\end{matrix} & (5)\end{matrix}$

$\begin{matrix}\begin{matrix}{b_{5} = {2{{Vp}/\left( {5\prod} \right)}\begin{Bmatrix}{{\sin\left( {{5{\prod{/3}}} - {5\phi}} \right)} - {\sin\; 5\phi} -} \\{{\sin\left( {5{\prod{{- 5}\phi}}} \right)} + {\sin\left( {{10{\prod{/3}}} + {5\phi}} \right)}}\end{Bmatrix}}} \\{= {2{{Vp}/{\left( {5\prod} \right)\begin{bmatrix}{{\sin\left\{ {\prod{- \left( {{{- 2}{\prod{/3}}} + {5\phi}} \right)}} \right\}} - {\sin\; 5\phi} -} \\{{\sin\left( {5{\prod{{- 5}\phi}}} \right)} + {\sin\left\{ {\prod{- \left( {{- {\prod{/3}}} - {5\phi}} \right)}} \right\}}}\end{bmatrix}}}}} \\{= {2{{Vp}/\left( {5\prod} \right)}\begin{Bmatrix}{{\sin\left( {{{- 2}{\prod{/3}}} + {5\phi}} \right)} -} \\{{2\sin\; 5\phi} + {\sin\left( {{- {\prod{/3}}} - {5\phi}} \right)}}\end{Bmatrix}}} \\{= {4{{Vp}/\left( {5\prod} \right)}\left\{ {{{\sin\left( {- {\prod{/2}}} \right)}{\cos\left( {{- {\prod{/6}}} - {5\phi}} \right)}} - {2\sin\; 5\phi}} \right\}}} \\{= {{- 4}{{Vp}/\left( {5\prod} \right)}\left\{ {{\cos\left( {{\prod{/6}} + {5\phi}} \right)} - {2\sin\; 5\phi}} \right\}}}\end{matrix} & (6)\end{matrix}$

By assigning the equations (3) through (6) to the equation (2), thefollowing equation is established:

$\begin{matrix}\begin{matrix}{{y(t)} = {\sum\limits_{n = 1}^{\infty}{b_{n}\cos\; n\;\omega\; t}}} \\{= {4{{Vp}/{\prod\begin{matrix}{{\left\{ {{\sin\left( {{\prod{/3}} - \phi} \right)} - {\sin\;\phi}} \right\}\cos\;\omega\; t} - {4{{Vp}/\left( {5\pi} \right)}}} \\{{\left\{ {{\cos\left( {{\prod{/6}} + {5\phi}} \right)} - {2\sin\; 5\phi}} \right\}\cos\; 5\;\omega\; t} + \ldots}\end{matrix}}}}} \\{= {{Vp}/{\prod\left\lbrack {4\begin{matrix}{{\left\{ {{\sin\left( {{\prod{/3}} - \phi} \right)} - {\sin\;\phi}} \right\}\cos\;\omega\; t} - {4/5}} \\{{\left\{ {{\cos\left( {{\prod{/6}} + {5\phi}} \right)} - {2\sin\; 5\phi}} \right)\cos\; 5\;\omega\; t} + \ldots}\end{matrix}} \right.}}}\end{matrix} & (7)\end{matrix}$

As obvious from the equation (7), the harmonic components of y(t) alwaysconsist of the fifth harmonic wave or more, which do not include thethird harmonic wave, irrespective of the value of ø. Accordingly, thecutoff frequency of the lowpass filter 22 is sufficient to be set to thefifth harmonic wave. In this way, since a relatively high cutofffrequency can be set, the inductance of the lowpass filter 22 may besmall. Therefore, as an inductor, one which is small, light andinexpensive may be used. Depending on the case, the lowpass filter 22itself may be omitted. In such a case, the driving circuit 20 consistssolely of the discontinuous rectangle wave oscillator 21, that is, itincludes an oscillating unit (FIG. 4) and a driving unit (FIG. 7) only.

Further, as obvious from the equation (4), the coefficient b₁ (that is,amplitude) of the fundamental wave can be changed by varying the phasedifference ø. Therefore, a DC-DC converter for making the voltage Vpvariable becomes unnecessary. The relationship between the phasedifference ø and the coefficient b₁ can be shown as follows, from theequation (4):

$\begin{matrix}\begin{matrix}{b_{1} = {4{{Vp}/{\prod\left\{ {{\sin\left( {{\prod{/3}} - \phi} \right)} - {\sin\;\phi}} \right\}}}}} \\{= {4{{Vp}/{\prod\left\{ {2\;{\cos\left( {\prod{/6}} \right)}{\sin\left( {{\prod{/6}} - \phi} \right)}} \right\}}}}} \\{= {\left( {4\left. \sqrt{}3 \right.{{Vp}/\prod}} \right){\sin\left( {{\prod{/6}} - \phi} \right)}}}\end{matrix} & (8)\end{matrix}$

The relationship of the equation (8) is shown in FIG. 2. As obvious fromFIG. 2, as the phase difference ø increases in a range between 0 andπ/6, the coefficient b₁ of the fundamental wave decreases almostlinearly. Note that in FIG. 2, the following lineb ₁′=(12√3Vp/π ²)(π/6−ø)  (9)is indicated for comparison.

Further, in the case of ø=0, from the equation (7), y(t) is given by thefollowing equation:

$\begin{matrix}\begin{matrix}{{y(t)} = {{Vp}/{\prod\left\lbrack \begin{matrix}{{4\left\{ {{\sin\;{\prod{/3}}} - {\sin\; 0}} \right\}\cos\;\omega\; t} - {4/5}} \\{{\left\{ {{\cos\;{\prod{/6}}} - {2\;\sin\; 0}} \right\}\cos\; 5\;\omega\; t} + \ldots}\end{matrix} \right.}}} \\{= {{Vp}/{\prod\left\lbrack {{2\left. \sqrt{}3 \right.\cos\;\omega\; t} - {2{\left. \sqrt{}3 \right./5}\cos\; 5\;\omega\; t} + \ldots} \right\rbrack}}} \\{= {\left( {2\left. \sqrt{}3 \right.{{Vp}/\prod}} \right)\begin{bmatrix}{{\cos\;\omega\; t} - {\left( {1/5} \right)\cos\; 5\;\omega\; t} + \left( {1/7} \right)} \\{{\cos\; 7\;\omega\; t} + \ldots + {\left( {1/11} \right)\cos\; 11\;\omega\; t} + \ldots}\end{bmatrix}}}\end{matrix} & (1)\end{matrix}$At this time, the coefficient b₁ of the fundamental wave becomes themaximum value 2√3Vp/π. Relating to the discontinuous rectangle waveshown by the equation [1] and the rectangle wave shown by theaforementioned formula [2], harmonic components are shown in FIG. 3.

FIG. 4 is a circuit diagram showing an example of an oscillating unit inthe driving circuit of FIG. 1. Hereinafter, explanation will be givenbased on this Fig. Note that “H level” means a high level, that is, ahigh voltage level (VDD), and “L level” means a low level, that is, alow voltage level (0).

The oscillating unit 30 includes a triangle wave generating circuit 31,a variable resistor 32, an inverting amplifier 33, comparators 34 a and34 b, inverters 35 a and 35 b, ring counters 361 to 364, differentiatingcircuits 371 a to 374 b, OR gates 381 to 384, and RSS-FFs (flip-flop) 39a and 39 b. The oscillating unit 30 outputs discontinuous rectangle wavegeneration signals V+ and V−.

The triangle wave generating circuit 31 is composed of a rectangle waveoscillator and an integrating circuit for example, and outputs atriangle wave voltage Vt1 to the inverting amplifier 33 and to apositive input terminal of the comparator 34 a. The variable resistor 32is, for example, a so-called “volume”. In the case of volume, a knob isturned manually, whereby an arbitral resistance value is set. Then, avoltage corresponding to the resistance value is outputted to negativeinput terminals of the comparators 34 a and 34 b as a reference voltageVr. The inverting amplifier 33 is composed of an operational amplifier331 and resistors 332 and 333, and inverts the triangle wave voltage Vt1and outputs it as a triangle wave voltage Vt2 to a positive inputterminal of the comparator 34 b.

The comparator 34 a compares the reference voltage Vr with the trianglewave voltage Vt1, and outputs an H level signal when Vr≦Vt1, and outputsan L level signal when Vr>Vt1. The comparator 34 b compares thereference voltage Vr with the triangle wave voltage Vt2, and outputs anH level signal when Vr≦Vt2, and outputs an L level signal when Vr>Vt2.An output signal of the comparator 34 a serves as a clock pulse CP1directly, and also becomes a clock pulse CP2 by being inverted by theinverter 35 a. Similarly, an output signal from the comparator 34 bserves as a clock pulse CP3 directly, and also becomes a clock pulse CP4by being inverted by the inverter 35 b.

The ring counter 361 includes an input terminal CLK1 to which the clockpulse CP1 is inputted, and three output terminals Q11 to Q13 from whichH level signals are outputted sequentially each time the clock pulse CP1is inputted. The configuration of the ring counters 362 to 364 issimilar to the ring counter 361. Further, the ring counters 361 to 364are provided with reset terminals (not shown) for obtainingsynchronization (or setting an initial value). The differentiatingcircuits 371 a to 374 b are composed of capacitors and resistors forexample, and convert output signals of the ring counters 361 to 364 totrigger signals of shorter pulse width, and output them to the OR gates381 to 384.

The input terminals of the OR gate 381 are connected with an outputterminal Q11 of the ring counter 361 and an output terminal Q31 of thering counter 363. The input terminals of the OR gate 382 are connectedwith an output terminal Q21 of the ring counter 362 and an outputterminal Q41 of the ring counter 364. The input terminals of the OR gate383 are connected with an output terminal Q13 of the ring counter 361and an output terminal Q32 of the ring counter 363. The input terminalsof the OR gate 384 are connected with an output terminal Q23 of the ringcounter 362 and an output terminal Q42 of the ring counter 364.

The RSS-FF 39 a includes an input terminal Sa for setting connected withthe output terminal of the OR gate 381, an input terminal Ra forresetting connected with the output terminal of the OR gate 382, and anoutput terminal Qa for outputting discontinuous rectangle wavegeneration signals V+. The RSS-FF 39 b includes an input terminal Sb forsetting connected with the output terminal of the OR gate 383, an inputterminal Rb for resetting connected with the output terminal of the ORgate 384, and an output terminal Qb for outputting discontinuousrectangle wave generation signals V−. Note that in a general RS-FF, itis prohibited that both of a set input S and a reset input R become “1”.On the other hand, in the RSS-FF, when both of a set input S and a resetinput R become “1”, the set input S is given priority, and “1” isoutputted.

FIGS. 5 and 6 are timing charts showing respective output signals in theoscillating unit of FIG. 4. Hereinafter, operation of the oscillatingunit will be explained based on FIGS. 4 to 6.

The variable resistor 32 is applied with a power supply voltage VDD, andthe reference voltage Vr is set to change from VDD/2 to VDD. On theother hand, the triangle wave voltage Vt1 is set to repeat from theminimum value 0 to the maximum value VDD cyclically, with T/3 being setas one cycle. Therefore, corresponding to the reference voltage Vr beingchanged from VDD/2 to VDD, the pulse width of the clock pulses CP1 toCP4 changes from T/6 to 0. That is, the phase difference ø varies fromT/12 to 0.

The ring counters 361 to 364 and RSS-FFs 39 a and 39 b perform positiveedge operation. First, in the ring counter 361, when the clock pulse CP1is inputted continuously from the input terminal CLK1, H level signalsare outputted sequentially from the output terminals Q11 to Q13. In thering counter 362, when the clock pulse CP2 is inputted continuously fromthe input terminal CLK2, H level signals are outputted sequentially fromthe output terminals Q21 to Q23. In the ring counter 363, when the clockpulse CP3 is inputted continuously from the input terminal CLK3, H levelsignals are outputted sequentially from the output terminals Q31 to Q33.In the ring counter 364, when the clock pulse CP4 is inputtedcontinuously from the input terminal CLK4, H level signals are outputtedsequentially from the output terminals Q41 to Q43.

The RSS-FF 39 a outputs discontinuous rectangle wave generation signalsV+ of the H level during a period from the time that an H level signalis outputted from the output terminal Q11 to the time that an H levelsignal is outputted from the output terminal Q21, and during a periodfrom the time that an H level signal is outputted from the outputterminal Q31 to the time that an H level signal is outputted from theoutput terminal Q41. The RSS-FF 39 b outputs discontinuous rectanglewave generation signals V− of the H level during a period from the timethat an H level signal is outputted from the output terminal Q32 to thetime that an H level signal is outputted from the output terminal Q42,and during a period from the time that an H level signal is outputtedfrom the output terminal Q13 to the time that an H level signal isoutputted from the output terminal Q23.

Note that when the phase difference ø=0, in the RSS-FF 39 a, H levelsignals are inputted simultaneously into the input terminal Sa forsetting and the input terminal Ra for resetting. At this time, theRSS-FF39 a gives priority to the H level signal of the input terminal Safor setting to thereby output discontinuous rectangle wave generationsignals V+ of the H level. Thereby, adjacent two discontinuous rectanglewave generation signals V+ of the H level becomes one without beinginterrupted. The operation of the RSS-FF 39 b is similar to this one.

FIG. 7 is a circuit diagram showing an example of a driving unit in thedriving circuit of FIG. 1. Hereinafter, explanation will be given basedon this Fig.

A driving unit 40 of the present invention consists of a so-calledH-type bridge circuit including transistors 41 and 42 of P-channel powerMOS and transistor 43 and 44 of N-channel power MOS. Based ondiscontinuous rectangle wave generation signals V+ and V−, thediscontinuous rectangle wave voltage Vs is applied to the firstelectrodes 12 and 13 of the piezoelectric transformer 10. The lowpassfilter 22 consisting of the inductor 23 is interposed between thedriving unit 40 and the piezoelectric transformer 10. To the output sideof the piezoelectric transformer 10, a load 50 is connected.

Gates of the transistors 41 and 43 are applied with discontinuousrectangle wave generation signals V+, and gates of the transistors 42and 44 are applied with discontinuous rectangle wave generation signalsV−. Therefore, when the discontinuous rectangle wave generation signalsV+ are in the H level, the transistor 41 becomes off and the transistor43 becomes on. In contrast, when the discontinuous rectangle wavegeneration signals V+ are in the L level, the transistor 41 becomes onand the transistor 43 becomes off. Similarly, when the discontinuousrectangle wave generation signals V− are in the H level, the transistor42 becomes off and the transistor 44 becomes on. In contrast, when thediscontinuous rectangle wave generation signals V− are in the L level,the transistor 42 becomes on and the transistor 44 becomes off.

Therefore, when both discontinuous rectangle wave generation signals V+and V− are in the L level, both transistors 43 and 44 become off,whereby the voltage applied to the primary electrodes 12 and 13 is 0.When the discontinuous rectangle wave generation signals V+ are in the Hlevel and the discontinuous rectangle wave generation signals V− are inthe L level, the transistors 42 and 43 become on and the transistors 41and 44 become off, whereby the voltage applied to the primary electrodes12 and 13 is Vp. In contrast, when the discontinuous rectangle wavegeneration signals V+ are in the L level and the discontinuous rectanglewave generation signals V− are in the H level, the transistors 42 and 43become off and the transistors 41 and 44 become on, whereby the voltageapplied to the primary electrodes 12 and 13 is −Vp. Therefore, thevoltage applied to the primary electrodes 12 and 13 based on thediscontinuous rectangle wave generation signals V+ and V− becomes adiscontinuous rectangle wave voltage Vs as shown in FIG. 6.

Needless to say, the present invention is not limited to theabove-described embodiment. For example, an oscillating unit and adriving unit may have other circuit configurations.

Availability of the Invention

According to the driving method and the driving circuit of apiezoelectric transformer of the present invention, by applying specificdiscontinuous rectangle wave voltage to primary electrodes of thepiezoelectric transformer, it is possible to eliminate the thirdharmonic wave outputted to the piezoelectric transformer. Thereby,reduction in size, reduction in weight, and reduction in price of thelowpass filter, or omission thereof can be achieved. Further, theamplitude of the fundamental wave can be varied by changing the phasedifference ø of the discontinuous rectangle wave, whereby a DC-DCconverter is not required. In other words, the inductance of a lowpassfilter can be reduced, and also the amplitude of the fundamental wavecan be changed without using a DC-DC converter.

1. A driving method of a piezoelectric transformer for applying voltageof a discontinuous rectangle wave, composed of a potential V₀, apotential V_(H) which is higher than the potential V₀ and a potentialV_(L) which is lower than the voltage V₀, to a primary electrode of thepiezoelectric transformer, wherein assuming one cycle is T, thediscontinuous rectangle wave has such a configuration that a potentialfrom a time 0 to a time ø is the potential V₀, from the time ø to a time(T/6−ø) is the potential V_(H), from the time (T/6−ø) to a time (T/3+ø)is the potential V₀, from the time (T/3+ø) to a time (T/2−ø) is thepotential V_(L), from the time (T/2−ø) to a time (T/2+ø) is thepotential V₀, from the time (T/2+ø) to a time (2T/3−ø) is the potentialV_(L), from the time (2T/3−ø) to a time (5T/6+ø) is the potential V₀,from the time (5T/6+ø) to a time (T−ø) is the potential V_(H), and fromthe time (T−ø) to a time T is the potential V₀, and a phase difference øis 0≦ø≦T/12.
 2. The driving method of the piezoelectric transformer asclaimed in claim 1, wherein a frequency component not less than a fifthharmonic wave, included in the discontinuous rectangle wave, is removedby using a lowpass filter, and then the discontinuous rectangle wave isapplied to the primary electrode of the piezoelectric transformer.
 3. Adriving method of a piezoelectric transformer for applying voltage of adiscontinuous rectangle wave, composed of a potential 0, a potential +1which is higher by a certain voltage than the potential 0 and apotential −1 which is lower by a certain voltage than the voltage 0, toa primary electrode of the piezoelectric transformer, wherein assumingone cycle is T, the discontinuous rectangle wave has such aconfiguration that a potential from a time 0 to a time ø is thepotential 0, from the time ø to a time (T/6−ø) is the potential +1, fromthe time (T/6−ø) to a time (T/3+ø) is the potential 0, from the time(T/3+ø) to a time (T/2−ø) is the potential −1, from the time (T/2−ø) toa time (T/2+ø) is the potential 0, from the time (T/2+ø) to a time(2T/3−ø) is the potential −1, from the time (2T/3−ø) to a time (5T/6+ø)is the potential 0, from the time (5T/6+ø) to a time (T−ø) is thepotential +1, and from the time (T−ø) to a time T is the potential 0,and a phase difference ø is 0≦ø≦T/12.
 4. The driving method of thepiezoelectric transformer as claimed in claim 3, wherein a frequencycomponent not less than a fifth harmonic wave, included in thediscontinuous rectangle wave, is removed by using a lowpass filter, andthen the discontinuous rectangle wave is applied to the primaryelectrode of the piezoelectric transformer.
 5. A driving circuit of apiezoelectric transformer, comprising a discontinuous rectangle waveoscillator for outputting voltage of a discontinuous rectangle wavewhich is applied to a primary electrode of the piezoelectrictransformer, wherein the discontinuous rectangle wave is composed of apotential V₀, a potential V_(H) which is higher than the potential V₀and a potential V_(L) which is lower than the voltage V₀, and assumingone cycle is T, the discontinuous rectangle wave has such aconfiguration that a potential from a time 0 to a time ø is thepotential V₀, from the time ø to a time (T/6−ø) is the potential V_(H),from the time (T/6−ø) to a time (T/3+ø) is the potential V₀, from thetime (T/3+ø) to a time (T/2−ø) is the potential V_(L), from the time(T/2−ø) to a time (T/2+ø) is the potential V₀, from the time (T/2+ø) toa time (2T/3−ø) is the potential V_(L), from the time (2T/3−ø) to a time(5T/6+ø) is the potential V₀, from the time (5T/6+ø) to a time (T−ø) isthe potential V_(H), and from the time (T−ø) to a time T is thepotential V₀, and a phase difference ø is 0≦ø≦T/12.
 6. The drivingcircuit of the piezoelectric transformer as claimed in claim 5, furthercomprising a lowpass filter, interposed between the rectangle waveoscillator and the piezoelectric transformer, for removing a frequencycomponent not less than a fifth harmonic wave, included in thediscontinuous rectangle wave outputted from the rectangle waveoscillator, and then outputting the discontinuous rectangle wave to thepiezoelectric transformer.
 7. A driving circuit of a piezoelectrictransformer, comprising a discontinuous rectangle wave oscillator foroutputting voltage of a discontinuous rectangle wave which is applied toa primary electrode of the piezoelectric transformer, wherein thediscontinuous rectangle wave is composed of a potential 0, a potential+1 which is higher by a certain voltage than the potential 0 and apotential −1 which is lower by a certain voltage than the voltage 0, andassuming one cycle is T, the discontinuous rectangle wave has such aconfiguration that a potential from a time 0 to a time ø is thepotential 0, from the time ø to a time (T/6−ø) is the potential +1, fromthe time (T/6−ø) to a time (T/3+ø) is the potential 0, from the time(T/3+ø) to a time (T/2−ø) is the potential −1, from the time (T/2−ø) toa time (T/2+ø) is the potential 0, from the time (T/2+ø) to a time(2T/3−ø) is the potential −1, from the time (2T/3−ø) to a time (5T/6+ø)is the potential 0, from the time (5T/6+ø) to a time (T−ø) is thepotential +1, and from the time (T−ø) to a time T is the potential 0,and a phase difference ø is 0≦ø≦T/12.
 8. The driving circuit of thepiezoelectric transformer as claimed in claim 7, further comprising alowpass filter, interposed between the rectangle wave oscillator and thepiezoelectric transformer, for removing a frequency component not lessthan a fifth harmonic wave, included in the discontinuous rectangle waveoutputted from the rectangle wave oscillator, and then outputting thediscontinuous rectangle wave to the piezoelectric transformer.
 9. Adriving method of a piezoelectric transformer comprising: a pulsegenerating step for generating a driving pulse of a fundamental wave byalternating polarities of rectangle pulses, in a double-humped shape,which form a unit; and a voltage applying step for applying voltagebased on the driving pulse of the fundamental wave generated in thepulse generating step to a primary electrode of the piezoelectrictransformer.
 10. The driving method of the piezoelectric transformer asclaimed in claim 9, wherein the driving pulse in which amplitude ischanged is generated by varying a phase difference with respect to thefundamental wave.
 11. The driving method of the piezoelectrictransformer as claimed in claim 9, wherein a frequency component notless than a fifth harmonic wave, included in the driving pulse, isfiltered.
 12. A driving circuit of a piezoelectric transformercomprising: an oscillating unit; and a driving unit, wherein theoscillating unit generates a driving pulse of a fundamental wave byalternating polarities of rectangle pulses, in a double-humped shape,which forms a unit, and the driving unit applies a voltage based on thedriving pulse of the fundamental wave generated by the oscillating unitto a primary electrode of the piezoelectric transformer.
 13. The drivingcircuit of the piezoelectric transformer as claimed in claim 12, whereinthe oscillating unit has a function of generating the driving pulse inwhich the amplitude is changed by varying the phase difference withrespect to the fundamental wave.
 14. The driving circuit of apiezoelectric transformer as claimed in claim 12, comprising a lowpassfilter for filtering a frequency component not less than a fifthharmonic wave included in the driving pulse.